1. Field of Invention
The present invention relates to DC-DC converters and more particularly to such converters employing gain hopping and pulse frequency modulation.
2. Related Art
Small electronic devices are commonly powered by batteries, which allow such devices to be portable. However, as battery use continues, the battery voltage drops, sometimes uniformly and sometimes in spurts, depending in part on the type of battery, the type of electronic device, and the frequency of device use. Such variations in the battery voltage may have undesirable effects on the operation of the electronic device powered by the battery. Consequently, DC-DC converters are commonly used to provide a constant and stable output supply voltage from the battery to the electronic device. DC-DC converters convert unregulated battery voltages to desired regulated output supply voltages to provide a constant power to the load or electronic device.
A fixed gain capacitor DC-DC boost converter may have a gain greater than or equal to one, while a fixed gain capacitor DC-DC buck converter may have a gain less than or equal to one. During the first part of a battery's life, when the battery voltage may be greater than the desired supply voltage, a buck converter can be used to provide an output voltage less than the battery voltage. During the second part of the battery's life, when the battery voltage may be less than the desired supply voltage, a boost converter can be used to provide an output voltage greater than the battery voltage. In addition to increasing or decreasing the battery voltage, voltage regulation is also required to precisely regulate the battery voltage at a constant desired voltage. A conventional method to regulate voltages in fixed gain capacitor DC-DC converters is to use pulse frequency modulation (PFM) or pulse skipping.
A typical regulated switched capacitor DC-DC converter 100, shown in FIG. 1, has a fixed gain switched capacitor circuit 110, a comparator 120, an AND gate 130, and a holding capacitor C coupled between ground and an output voltage V.sub.o. An input voltage V.sub.i, i.e. from a battery (not shown), is provided to fixed gain switched capacitor circuit 110 with gain G. Such switched capacitor circuits 110, which are well known in the art, typically comprise a configuration of switches and capacitors with a control circuit for turning the switches on and off. The operation of the switches allows the capacitors to be configured and reconfigured such that selected capacitors are charged and discharged to convert input voltage V.sub.i to output voltage V.sub.o. For example, switched capacitor circuit 110 having a gain of 1.5 converts V.sub.i to an output of 1.5*V.sub.i (assuming no load).
Comparator 120, such as an operational amplifier (op amp), compares V.sub.o with a desired output voltage V.sub.od and provides an amplified difference signal to one input of AND gate 130, with the other input being a signal from an oscillator. If V.sub.o &lt;V.sub.od, then the input must still be boosted. Consequently, clock signal CLK from AND gate 130 directs switched capacitor circuit 110 to again boost the input signal. However, if V.sub.o .gtoreq.V.sub.od, indicating sufficient output supply voltage, then CLK directs switched capacitor circuit 110 to skip the boosting operation for the current clock cycle. As a result, V.sub.o is regulated by modulating the frequency of boosting by switched capacitor circuit 110 such that during one time frame, the number of clock cycles at which the switched capacitor circuit operates increases as V.sub.i decreases (assuming a constant load (not shown)), and vice versa.
Similarly, for switched capacitor circuits with gains less than or equal to one, i.e., voltage, divider circuits, during the first part of the battery's life, the input voltage V.sub.i is converted to an output voltage lower than V.sub.i in order to maximize conversion efficiency and prevent unnecessary expenditure of battery power. During the second part of the battery's life, the battery voltage V.sub.i is used directly as the output voltage V.sub.o. As such, PFM again regulates V.sub.o.
One problem with single fixed gain buck or boost topologies is that conversion efficiency can suffer at certain input voltages. Efficiency can be approximated as follows: ##EQU1##
For example, if V.sub.od is 5 volts, and the gain G of the switched capacitor circuit is 3, then efficiency is maximized when V.sub.i is 5/3 volts. However, if V.sub.i is greater than 5/3 volts, then the output voltage provided from a gain of 3 is greater than what is required, thereby reducing efficiency.
Thus, to increase efficiency, a DC-DC converter chooses, based on the input voltage, one of a multiple of switched capacitor gains to maximize conversion efficiency. For example, in FIG. 2, a regulated 5V DC-DC converter 200 (MAX619) from Maxim Integrated Products of Sunnyvale, Calif. acts as either a voltage doubler or voltage tripler, depending on the input voltage. Converter 200 differs from, converter 100 of FIG. 1 in that switched capacitor circuit 210 is configured so that by operation of the switches, two gains, G.sub.1 and G.sub.2, are possible instead of just one. Such topologies are well known in the art. Another difference is that an analog-to-digital (A/D) converter 220 coupled between input voltage V.sub.i and switched capacitor circuit 210 converts the analog input voltage V.sub.i to one of three ranges. Thus, to maximize efficiency over the entire range of input voltages (MAX619 is intended for operation with an input of 2.0 to 3.6 V to provide an output of 5V), the switched capacitor circuit acts as a voltage doubler when V.sub.i ranges from 3.0 to 3.6 V, a voltage tripler when V.sub.i ranges from 2.0 to 2.5 V, and alternates between a voltage doubler and voltage tripler when V.sub.i ranges from 2.5 to 3.0 V.
While DC-DC converters commonly use the input voltage to select an appropriate gain, using the input voltage to select the gain can be problematic when the output impedance is large. Selecting a gain such that G*V.sub.i .gtoreq.V.sub.od does not ensure that voltage supplied to the load will always meet or exceed the desired voltage. For example, if a large voltage drop exists across the output impedance, which includes capacitor and switch impedances, the input, and the frequency of switching, then the selected gain may be insufficient to provide the desired output voltage. Therefore, the gain must be chosen to take into account the drop across the output impedance for a maximum load current, thereby reducing efficiency.
Accordingly, a DC-DC converter is desired which overcomes the problems discussed above with respect to conventional DC-DC converters.